Log Message:
py-test-xdist: updated to 1.21.0

pytest-xdist 1.21.0:

Deprecations and Removals
Drop support for EOL Python 2.6.

Features
New --dist=loadfile option which load-distributes test to workers grouped by the \ 
file the tests live in.

Bug Fixes
Fix accidental mutation of test report during serialization causing longrepr \ 
string-ification to break.